1. Field of the Invention
The present invention relates to a brake abnormality diagnosis method for an industrial robot or a machine tool having a brake-equipped motor, and to a brake abnormality diagnosis device which implements such a method.
2. Description of Related Art
Industrial robots and machine tools having a gravity axis (hereinafter, in some cases industrial robots and machine tools are together referred to as a “robot”) include a brake-equipped motor. If the motor is interrupted, the brake is used in order to maintain the posture of the robot.
However, if adherent matter such as grease or oil adheres to a friction plate of the brake provided in the motor, the braking torque of the brake decreases. If there is a substantial decrease in the braking torque, it is no longer possible for the robot to maintain the posture thereof when the excitation of the motor is interrupted.
If this kind of situation occurs when, for example, gravitational force is acting on an output axis of the motor, in other words when the axis is a gravity axis, the gravity axis may drop. Therefore, it is desirable for the decrease in the braking torque of the brake to be regularly diagnosed.
In Japanese Unexamined Patent Publication No. 06-246674, coasting distance of a robot from a generation of an emergency stop signal for the robot to an emergency stop of the robot is detected, and it is determined that there is an abnormality in the brake if this coasting distance is greater than a reference distance. Furthermore, in Japanese Unexamined Patent Publication No. 06-246674, when power is supplied, if torque corresponding to a predetermined braking-torque reference value is generated in the motor of the robot and the rotation of the motor is equal to or greater than a reference value, it is determined that there is an abnormality in the brake.
Usually, once the robot is powered on, the robot is repeatedly activated and stopped without the power supply being interrupted all day long. There are cases where there is a gradual decrease in the braking torque of the brake due to adherent matter such as grease or oil adhering to the friction plate of the brake as a result of the robot being repeatedly activated and stopped.
In Japanese Unexamined Patent Publication No. 06-246674, brake abnormality determination is performed when the robot is made to perform an emergency stop and when the robot is powered on. Therefore, even though brake abnormalities can be discovered at times such as when the robot is made to perform an emergency stop, there are cases when the decrease in braking torque has progressed to a considerable extent at the time of discovering an abnormality. Therefore, if there is a substantial decrease in the braking torque and the axis is a gravity axis, the gravity axis may drop.
The present invention has been devised in light of such circumstances, and an object of the present invention is to provide a brake abnormality diagnosis method with which a dropping of a gravity axis due to a decrease in a braking torque of a brake can be prevented, and to provide a brake abnormality diagnosis device which implements such a method.